Due to the chemical constitution of mud and especially clay, the particles are easily bound together, as we have seen in Sect. 2.6.2. Organic matter is also easily bound to mud particles. This is one of the main reasons why delta areas are so fertile and thus one of the main reasons why people settle in those low-lying, rather dangerous areas. The fertile coastal areas are also good habitats for flora and fauna.
Unfortunately, contaminants are also easily bound by the mud particles. At certain (low) concentrations, naturally occurring elements (i.e. copper and zinc) may have beneficial effects. But with increasing concentration, contaminants can become toxic and cause damage to the natural environment. An increase in concentration can occur due to fluvial inputs in the coastal system, but also as a result of a disturbance of a contaminated bed caused by, for example, dredging.
During dredging operations, extremely dangerous contaminants such as heavy metals and PCB may be part of the dredged material. Highly contaminated mud has to be treated as chemical waste. This can have large consequences for the costs of a dredging project in a harbour. The dredged material must be transported to special depots as for example the ‘Slufter’ near the Port of Rotterdam (see Fig. 6.18) and isolated from the environment.
For the construction of large land reclamation projects, huge volumes of sand are required. These volumes are often dredged at the sea bottom as close as possible to the construction site; so in the most vulnerable area close to the coast. Let us assume that for a very large project 500 million \(m^3\) of sand is required. The sand can be dredged in the open sea, but the bottom contains 2% silt (a quite normal content for natural seabeds). During the dredging operation with hopper dredges, the bed material is pumped into the hopper. The sand remains in the hopper; the fines (silt) return to the sea with the overflow. Dredging 500 million \(m^3\) for the land reclamation means that about 10 million \(m^3\) of silt is mobilised in a relatively short period of time. This might result in a heavy additional silt burden for the receiving sea system. Detrimental effects are likely.